[GHC] #12485: -package-db flags now need to be sorted by dependency order

[GHC]

#12485: -package-db flags now need to be sorted by dependency order
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        Reporter:  niteria           |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Package system    |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  phab:D2450,
       Wiki Page:                    |  phab:D2514
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Comment (by simonmar):

 @ezyang, I'd like to find a solution to this, because otherwise we need
 some pretty horrible hacks at our end to keep our package DBs ordered.

 I read your long description in comment:9 (twice!) but I'm afraid I still
 don't understand what change required this new ordering constraint on
 package DBs.  The original commit
 (39b71e81ec1044518f065d0055676d713521e483) didn't explain the motivation
 either.

 My recollection of the algorithm before was that we throw all the packages
 together, resolve any clashes by using the package DB ordering, and then
 throw away packages whose deps are not satisfied, recursively.  Could you
 explain what went wrong that caused GHC to not bootstrap?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12485#comment:15>
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